The present invention relates to a fluid control circuit for regulating communication between a source of fluid under pressure and a hydraulic motor. More particularly, the invention relates to such a circuit adapted for regulating high pressure fluid flow as is commonly required in the operation of various machines such as earthmoving equipment. The invention especially contemplates improved operation of a control valve within the circuit and improved response of the motor to manipulation of the control valve.
In hydraulic circuits of the type noted above, the source is in communication with an lnlet of the control valve with a relief valve being employed to provide load responsive operating fluid flow and pressure when the control valve is conditioned to communicate fluid from its inlet to the hydraulic motor. Otherwise, fluid entering the inlet chamber from the source is returned to a drain with only minimum resistance provided by a spring within the relief valve. Such an inlet relief valve is conditioned to commence pressure modulation by fluid signals communicated to the spring chamber end of the relief valve from the control valve.
Load-compensating, flow control valves of this type are employed because the source or pump need not operate against a substantial resistance when fluid pressure is not being communicated to the motor. Also, since the modulated pressure established by the inlet relief valve is a function of load pressure and the pressure rating of its biasing spring rather than an arbitrarily fixed pressure, the control valve spool experiences reduced flow forces which tend to facilitate its manipulation. Such a control circuit is described, for example, in U.S. patent application, Ser. No. 211,333, filed Dec. 23, 1971 and assigned to the assignee of the present invention, now U.S. Pat. No. 3,847,180, issued Nov. 12, 1974. Additional prior art references relating to load compensating, flow control circuits include U.S. Pat. Nos. 3,260,325; 3,566,749; and 3,693,506, all of which are assigned to the assignee of the present invention. Additional prior art references include U.S. Pat. Nos. 2,941,547 and 3,718,159.
Within the prior art control circuits, it is common to design the main control valve with close manufacturing tolerances so that various passages within the control circuit open and close at desirably the same instant. Further, the inlet relief valve is commonly responsive to a fluid signal or signals also communicated from the control valve at the same time.
Such design criteria, relating to "timing" of the valve components within the control circuit may in turn lead to problems where various passages of the control circuit are undesirably blocked or, on the other hand, where the passages may experience undesirable cross-flow or leakage resulting in the dropping of a load. Generally, it appears that undesirable cross-flow has been considered more of a problem within such control circuits. Accordingly, the control valves have commonly been designed with a "dead band" where the passages leading to the inlet relief valve, for example, are neither in positive communication with a pressurized passage nor with a fluid drain as the control valve is being moved from a neutral position into a position for operating the motor. Such a condition produces a lack of positive control of the inlet chamber pressure.
Furthermore, pressure within the inlet chamber tends to be modulated toward a desired operating pressure commencing at the same instant that the inlet chamber enters into communication with a service chamber. Accordingly, at the instant when one of the service chambers is placed in communication with the inlet chamber, it is possible that fluid pressure within the inlet chamber may not exceed that in the service chamber so that fluid flow to the service chamber from the inlet passage may experience a slight but nevertheless undesirable delay.